Abstract:

A retardation film comprising a monolayer film having a birefringent chain
polymer orientated. The polymer has a repeating unit of the structure of
the general formula: (V) wherein R3 is a hydrogen atom or
C1-C8 alkyl; each of R4 and R8 independently is a
hydrogen atom, C1-C4 linear or branched alkyl, C1-C4
linear or branched alkoxyl, C1-C4 linear or branched
thioalkoxyl, halogen, nitro, amino, hydroxyl or thiol (with the proviso
that R4 and R8 are not simultaneously hydrogen atoms); and each
of R5, R6 and R7 independently is a hydrogen atom or a
substituent. This retardation film can be a relatively thin layer, and
exhibits such a wavelength dispersion that with respect to light rays of
wavelength λ of approximately the entire visible region, the phase
contrast is about λ/2 or about λ/4.

Claims:

1. A retardation film consisting of a single-layer film in which a
birefringent chain polymer is oriented, the polymer having as a side
chain (a), a group represented by the following general formula (I), the
retardation film having a smaller in-plane retardation at a shorter
wavelength lying within a range of at least 450 to 650 nm, and having a
larger in-plane retardation at a longer wavelength lying within a range
of at least 450 to 650 nm:wherein two oxygen atoms are bonded to atoms
constituting a main chain, respectively; and R1 and R2 each
independently represent a hydrogen atom, an alkyl group having 1 to 8
carbon atoms, or an aromatic group (where at least either R1 or
R2 is an aromatic group, and the aromatic group(s) represented by
R1 or/and R2 is (are) arranged in a direction such that the
planar structure thereof is substantially orthogonal to a virtual line
obtained by connecting the two oxygen atoms).

2. A retardation film consisting of a single-layer film in which a
birefringent chain polymer is oriented, the polymer having as a side
chain (a), at least either a group represented by the following general
formula (II) or a group represented by the following general formula
(III) the retardation film having a smaller in-plane retardation at a
shorter wavelength lying within a range of at least 450 to 650 nm, and
having a larger in-plane retardation at a longer wavelength living within
a range of at least 450 to 650 nm:wherein two oxygen atoms are bonded to
atoms constituting a main chain, respectively; R3 represents a
hydrogen atom or an alkyl group having 1 to 8 carbon atoms; R4 and
R8 each independently represent a hydrogen atom, a linear or
branched alkyl group having 1 to 4 carbon atoms, a linear or branched
alkoxyl group having 1 to 4 carbon atoms, a linear or branched
thioalkoxyl group having 1 to 4 carbon atoms, halogen, a nitro group, an
amino group, a hydroxyl group, or a thiol group (where R4 and
R8 are not simultaneously hydrogen atoms); and R5, R6, and
R7 each independently represent a hydrogen atom or a
substituent,wherein two oxygen atoms are bonded to atoms constituting a
main chain, respectively; R3 represents a hydrogen atom or an alkyl
group having 1 to 8 carbon atoms; and A represents a naphthyl group which
may have a substituent, an anthranyl group which may have a substituent,
or a phenanthrenyl group which may have a substituent (where one or more
carbon atoms constituting the naphthyl, anthranyl, or phenanthrenyl group
may be substituted by one or more nitrogen atoms).

3. The retardation film according to claim 1 or 2, wherein the polymer has
a part where structural units each having a structure, in which the side
chain (a) is bonded to constituent atoms of the main chain thereof, are
adjacent to each other.

4. The retardation film according to claim 1 or 2, wherein the amount of
the side chain (a) contained in the polymer is 1 to 50 mol % with respect
to the total amount of side chains of the polymer.

5. The retardation film according to claim 1 or 2, wherein the polymer
has, in addition to the side chain (a), a hydroxyl group as a side chain
(b).

6. The retardation film according to claim 5, wherein the polymer has, in
addition to the side chains (a) and (b), a group represented by the
following general formula (IV) as a side chain (c):wherein R9
represents a hydrogen atom or a linear, branched or cyclic alkyl group
(where one or more carbon atoms of the alkyl group may be substituted by
one or two or more non adjacent oxygen atoms).

7. The retardation film according to claim 6, wherein the amount of the
side chain (a), the amount of the side chain (b), and the amount of the
side chain (c) contained in the polymer are 1 to 50 mol %, 5 to 95 mol %,
and 1 to 90 mol %, respectively, with respect to the total amount of side
chains of the polymer.

8. A retardation film consisting of a single-layer film in which a
birefringent chain polymer is oriented, the polymer having as a repeating
unit (A), at least either a structure represented by the following
general formula (V) or a structure represented by the following general
formula (VI), the retardation film having a smaller in-plane retardation
at a shorter wavelength lying within a range of at least 450 to 650 nm,
and having a larger in-plane retardation at a loner wavelength lying
within a range of at least 450 to 650 nm:wherein R3 represents a
hydrogen atom or an alkyl group having 1 to 8 carbon atoms; R4 and
R8 each independently represent a hydrogen atom, a linear or
branched alkyl group having 1 to 4 carbon atoms, a linear or branched
alkoxyl group having 1 to 4 carbon atoms, a linear or branched
thioalkoxyl group having 1 to 4 carbon atoms, halogen, a nitro group, an
amino group, a hydroxyl group, or a thiol group (where R4 and
R8 are not simultaneously hydrogen atoms); and R5, R6, and
R7 each independently represent a hydrogen atom or a
substituent.wherein R3 represents a hydrogen atom or an alkyl group
having 1 to 8 carbon atoms; and A represents a naphthyl group which may
have a substituent, an anthranyl group which may have a substituent, or a
phenanthrenyl group which may have a substituent (where one or more
carbon atoms constituting the naphthyl, anthranyl, or phenanthrenyl group
may be substituted by one or more nitrogen atoms).

9. The retardation film according to claim 8, wherein the repeating unit
(A) of the polymer is obtained by acetalization of hydroxyl groups of
polyvinyl alcohol with aromatic aldehyde.

10. The retardation film according to claim 8, wherein the polymer has, in
addition to the repeating unit (A), a repeating unit (B) represented by
the following general formula (VII), and wherein the repeating units (A)
and (B) may be arranged in either block or random fashion:

11. The retardation film according to claim 10, wherein the polymer has,
in addition to the repeating units (A) and (B), a repeating unit (C)
represented by the following general formula (VIII), and wherein the
repeating units (A) to (C) may be arranged in either block or random
fashion:wherein R9 represents a hydrogen atom or a linear, branched,
or cyclic alkyl group having 1 to 12 carbon atoms (where one or more
carbon atoms of the alkyl group may be substituted by one or two or more
non-adjacent oxygen atoms).

12. A retardation film consisting of a single-layer film in which a
birefringent chain polymer is oriented, the polymer having a structure
represented by the following general formula (IX), the retardation film
having a smaller in-plane retardation at a shorter wavelength lying
within a range of at least 450 to 650 nm, and having a larger in-plane
retardation at a loner wavelength lying within a range of at least 450 to
650 nm:wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

13. A retardation film consisting of a single-layer film in which a
birefringent chain polymer is oriented, the polymer having a structure
represented by the following general formula (X), the retardation film
having a smaller in-plane retardation at a shorter wavelength lying
within a range of at least 450 to 650 nm, and having a larger in-plane
retardation at a longer wavelength lying within a range of at least 450
to 650wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

14. A polymer represented by the following general formula (XII).wherein l
is 5 to 30 mol %, m is 20 to 60 mol %, n is 20 to 60 mol %, and o is 1 to
55 mol %.

15. An optical film having a laminate structure comprising the retardation
film according to any one of claims 1, 2, 8, 12, and

16. An image display device comprising the retardation film according to
any one of claims 1, 2, 8, 12, and 13.

17. An image display device comprising the optical film according to claim
15.

18-25. (canceled)

Description:

TECHNICAL FIELD

[0001]The present invention relates to a retardation film, a novel
polymer, an optical film using the retardation film, and an image display
device.

BACKGROUND ART

[0002]Retardation films are optical members to be used for obtaining
various kinds of polarized light such as linearly polarized light,
circularly polarized light, and elliptically polarized light. As such
retardation films, a λ/4 plate whose retardation is 1/4 of a
wavelength λ and a λ/2 plate whose retardation is 1/2 of a
wavelength λ are known. The λ/4 plate has an optical function
of converting linearly polarized light to circularly polarized light, and
the λ/2 plate has an optical function of rotating the plane of
polarization of linearly polarized light by 90°. Generally, such
retardation films designed to act as a λ/4 plate or a λ/2
plate for light having a specific wavelength cannot perform the same
function for light having any other wavelength. For example, a
retardation film designed to act as a λ/4 plate for light having a
wavelength of 550 nm cannot perform the same function for light having a
wavelength of 450 nm or 650 nm. That is, retardation films generally
exhibit wavelength dispersion characteristics such that the retardation
thereof depends on wavelength. For example, it is generally known that a
polymer film exhibits wavelength dispersion characteristics such that the
retardation thereof is larger at a shorter wavelength and is smaller at a
longer wavelength.

[0003]When white light being a composite of various wavelengths of visible
light rays enters a retardation film exhibiting such wavelength
dispersion characteristics, the wavelength dispersion characteristics
cause a problem that the form of polarization of light rays greatly
varies depending on their respective wavelengths and therefore
distribution of polarization state occurs, so that the incident white
light is converted to colored light.

[0004]In order to overcome such a problem, Japanese Patent Laid-open No.
Hei 10-239518 has proposed a retardation film having a wavelength
dispersion value α of less than 1, which is obtained by laminating
together two or more birefringent media having different wavelength
dispersion values α(α=Δn(450 nm)/Δn(650 nm)) in
such a manner that their slow axes intersect at right angles. Japanese
Patent Laid-open No. Hei 10-239518 describes that such a retardation film
has the effect of giving a constant optical retardation to any wavelength
lying within a visible light wavelength range (that is, such a
retardation film provides a constant optical retardation irrespective of
wavelength) so that white light can be easily obtained.

[0005]However, since the retardation film disclosed in Japanese Patent
Laid-open No. Hei 10-239618 is a laminated product comprising two or more
birefringent media, it requires a process for laminating these
birefringent media and bonding them together. In addition to that, it is
also necessary to select two or more birefringent materials and an
adhesive for bonding two or more birefringent media. Further, since such
a retardation film having a laminate structure becomes relatively thick,
it is not suitable for use in, for example, liquid crystal displays
required to be smaller in thickness.

[0006]It is therefore an object of the present invention to provide a
retardation film which can be relatively small in thickness and which
exhibits wavelength dispersion characteristics such that retardation for
light having a wavelength of λ lying within almost the entire
visible light wavelength range from 400 to 700 nm is about λ/2 or
λ/4, a novel polymer that can be suitably used as a material for
forming such a retardation film, an optical film, and an image display
device.

DISCLOSURE OF THE INVENTION

[0007]The present inventors have intensively studied various materials to
solve the problems described above, and as a result, they have found that
the problems can be solved by using a chain polymer obtained by
introducing a specific side chain into a main chain.

[0008]In order to achieve the above object, the present invention provides
a retardation film (1) consisting of a single-layer film in which a
birefringent chain polymer is oriented, the chain polymer having as a
side chain (a), a group represented by the following general formula (I):

[0009]wherein two oxygen atoms are bonded to atoms constituting a main
chain, respectively; and R1 and R2 each independently represent
a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an
aromatic group (where at least either R1 or R2 is an aromatic
group, and the aromatic group(s) represented by R1 or/and R2 is
(are) arranged in a direction such that the planar structure thereof is
substantially orthogonal to a virtual line obtained by connecting the two
oxygen atoms).

[0010]As described above, the retardation film of the present invention is
consisted of a single-layer film, and therefore can be smaller in
thickness than a conventional retardation film. Further, this retardation
film gives a retardation of, for example, about λ/2 or λ/4 to
light having a wavelength of λ lying within almost the entire
visible light wavelength range from 400 to 700 nm, thereby providing
substantially the same form of polarization at any wavelength from 400 to
700 nm. Therefore, even when white light enters this retardation film,
the white light is not converted to colored polarized light, that is,
white polarized light is obtained.

[0011]The present invention also provides a retardation film (2)
consisting of a single-layer film in which a birefringent chain polymer
is oriented, the polymer having as a side chain (a), at least either a
group represented by the following general formula (II) or a group
represented by the following general formula (III):

[0012]wherein two oxygen atoms are bonded to atoms constituting a main
chain, respectively; R3 represents a hydrogen atom or an alkyl group
having 1 to 8 carbon atoms; R4 and R8 each independently
represent a hydrogen atom, a linear or branched alkyl group having 1 to 4
carbon atoms, a linear or branched alkoxyl group having 1 to 4 carbon
atoms, a linear or branched thioalkoxyl group having 1 to 4 carbon atoms,
halogen, a nitro group, an amino group, a hydroxyl group, or a thiol
group (where R4 and R8 are not simultaneously hydrogen atoms);
and R5, R6, and R7 each independently represent a hydrogen
atom or a substituent,

[0013]wherein two oxygen atoms are bonded to atoms constituting a main
chain, respectively; R3 represents a hydrogen atom or an alkyl group
having 1 to 8 carbon atoms; and A represents a naphthyl group which may
have a substituent, an anthranyl group which may have a substituent, or a
phenanthrenyl group which may have a substituent (where one or more
carbon atoms constituting the naphthyl, anthranyl, or phenanthrenyl group
may be substituted by one or more nitrogen atoms).

[0014]The present invention also provides a retardation film (3) according
to (1) or (2) described above, wherein the polymer has a part where
structural units each having a structure, in which the side chain (a) is
bonded to constituent atoms of the main chain of the polymer, are
adjacent to each other.

[0015]The present invention also provides a retardation film (4) according
to any one of (1) to (3) described above, which has a smaller in-plane
retardation at a shorter wavelength lying within a range of at least 450
to 650 nm, and has a larger in-plane retardation at a longer wavelength
lying within a range of at least 450 to 650 nm.

[0016]The present invention also provides a retardation film (5) according
to any one of (1) to (4) described above, wherein the amount of the side
chain (a) contained in the polymer is 1 to 50 mol % with respect to the
total amount of side chains of the polymer.

[0017]The present invention also provides a retardation film (6) according
to any one of (1) to (5) described above, wherein the polymer has, in
addition to the side chain (a), a hydroxyl group as a side chain (b).

[0018]The present invention also provides a retardation film (7) according
to (6) described above, wherein the polymer has, in addition to the side
chains (a) and (b), a group represented by the following general formula
(IV) as a side chain (c):

[0019]wherein R9 represents a hydrogen atom or a linear, branched or
cyclic alkyl group (where one or more carbon atoms of the alkyl group may
be substituted by one or two or more non-adjacent oxygen atoms).

[0020]The present invention also provides a retardation film (8) according
to (7) described above, wherein the amount of the side chain (a), the
amount of the side chain (b), and the amount of the side chain (c)
contained in the polymer are 1 to 50 mol %, 5 to 95 mol %, and 1 to 90
mol %, respectively, with respect to the total amount of side chains of
the polymer.

[0021]The present invention also provides a retardation film (9)
consisting of a single-layer film in which a birefringent chain polymer
is oriented, the polymer having as a repeating unit (A), at least either
a structure represented by the following general formula (V) or a
structure represented by the following general formula (VI):

[0022]wherein R3 represents a hydrogen atom or an alkyl group having
1 to 8 carbon atoms; R4 and R8 each independently represent a
hydrogen atom, a linear or branched alkyl group having 1 to 4 carbon
atoms, a linear or branched alkoxyl group having 1 to 4 carbon atoms, a
linear or branched thioalkoxyl group having 1 to 4 carbon atoms, halogen,
a nitro group, an amino group, a hydroxyl group, or a thiol group (where
R4 and R8 are not simultaneously hydrogen atoms); and R5,
R6, and R7 each independently represent a hydrogen atom or a
substituent,

[0023]wherein R3 represents a hydrogen atom or an alkyl group having
1 to 8 carbon atoms; and A represents a naphthyl group which may have a
substituent, an anthranyl group which may have a substituent, or a
phenanthrenyl group which may have a substituent (where one or more
carbon atoms constituting the naphthyl, anthranyl, or phenanthrenyl group
may be substituted by one or more nitrogen atoms).

[0024]The present invention also provides a retardation film (10)
according to (9) described above, wherein the repeating unit (A) of the
polymer is obtained by acetalization of hydroxyl groups of polyvinyl
alcohol with aromatic aldehyde.

[0025]The present invention also provides a retardation film (11)
according to (9) or (10) described above, wherein the polymer has, in
addition to the repeating unit (A), a repeating unit (B) represented by
the following general formula (VII), and wherein the repeating units (A)
and (B) may be arranged in either block or random fashion:

[0026]The present invention also provides a retardation film (12)
according to (11) described above, wherein the polymer has, in addition
to the repeating units (A) and (B), a repeating unit (C) represented by
the following general formula (VIII), and wherein the repeating units (A)
to (C) may be arranged in either block or random fashion:

[0027]wherein R9 represents a hydrogen atom or a linear, branched, or
cyclic alkyl group having 1 to 12 carbon atoms (where one or more carbon
atoms of the alkyl group may be substituted by one or two or more
non-adjacent oxygen atoms).

[0028]The present invention also provides a retardation film (13)
consisting of a single-layer film in which a birefringent chain polymer
is oriented, the polymer having a structure represented by the following
general formula (IX):

[0029]wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

[0030]The present invention also provides a retardation film (14)
consisting of a single-layer film in which a birefringent chain polymer
is oriented, the polymer having a structure represented by the following
general formula (X):

[0031]wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

[0032]The present invention also provides a polymer comprising a repeating
unit represented by the following general formula (V'):

[0033]wherein R3 represents a hydrogen atom or an alkyl group having
1 to 8 carbon atoms; R4 and R8 each independently represent a
linear or branched alkyl group having 1 to 4 carbon atoms, a linear or
branched alkoxyl group having 1 to 4 carbon atoms, a linear or branched
thioalkoxyl group having 1 to 4 carbon atoms, halogen, a nitro group, an
amino group, a hydroxyl group, or a thiol group; and R5, R6,
and R7 each independently represent a hydrogen atom or a
substituent.

[0034]The present invention also provides a polymer comprising a repeating
unit represented by the general formula (V') and a repeating unit
represented by the following general formula (VIII), the repeating units
being arranged in either block or random fashion:

[0035]wherein R9 represents a hydrogen atom or a linear, branched, or
cyclic alkyl group having 1 to 12 carbon atoms (where one or more carbon
atoms of the alkyl group may be substituted by one or two or more
non-adjacent oxygen atoms).

[0036]In the polymer having a repeating unit represented by the general
formula (V'), it is preferred that R4 and R8 are each
independently a linear or branched alkyl group having 1 to 4 carbon atoms
or a chlorine atom.

[0037]Further, in the polymer having a repeating unit represented by the
general formula (V'), it is also preferred that R3, R5, and
R7 are each a hydrogen atom and R4 and R8 are each a
methyl group.

[0038]The present invention also provides a polymer represented by the
following general formula (IX):

[0039]wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

[0040]The present invention also provides a polymer represented by the
following general formula (X):

[0041]wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

[0042]The present invention also provides a polymer represented by the
following general formula (XI):

[0043]wherein l is 5 to 30 mol %, m is 20 to 80 mol %, and n is 1 to 70
mol %.

[0044]The present invention also provides a polymer represented by the
following general formula (XII):

[0045]wherein l is 5 to 30 mol %, m is 20 to 60 mol %, n is 20 to 60 mol
%, and o is 1 to 55 mol %.

[0046]The present invention also provides an optical film (15) having a
laminate structure comprising the retardation film according to any one
of (1) to (14) described above.

[0047]The present invention also provides an image display device
comprising the retardation film according to any one of (1) to (14)
described above or the optical film (15) described above.

BRIEF DESCRIPTION OF THE DRAWING

[0048]FIG. 1 is a graph which shows a result of measurement by polarized
IR.

BEST MODE FOR CARRYING OUT THE INVENTION

[0049]The present inventors have found that a single-layer film in which a
chain polymer having a side chain (a), in which an aromatic group is
introduced into --OCO--, is oriented has a smaller in-plane retardation
at a shorter wavelength lying within a visible light wavelength range of
at least 450 to 650 nm. The present invention entirely utilizes such
characteristics of the single-layer film to provide a retardation film
having a relatively small thickness and exhibiting a predetermined
retardation at wavelengths lying in almost the entire visible light
wavelength range of 400 to 700 nm, and a novel polymer which can be
suitably used for forming such a retardation film.

[0050]Hereinbelow, the present invention will be described more
specifically. It is to be noted that in this specification, the
characteristics of the retardation film of the present invention such
that the in-plane retardation thereof is smaller at a shorter wavelength
lying within a visible light wavelength range is also referred to as
"reverse wavelength dispersion characteristics".

[0051]The present invention provides a retardation film consisting of a
single-layer film in which a birefringent chain polymer is oriented, the
polymer having as a side chain (a), a group represented by the following
general formula (I):

[0052]wherein two oxygen atoms are bonded to atoms constituting a main
chain, respectively; and R1 and R2 each independently represent
a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, or an
aromatic group (where at least either R1 or R2 is an aromatic
group, and the aromatic group(s) represented by R1 or/and R2 is
(are) arranged in a direction such that the planar structure thereof is
substantially orthogonal to a virtual line obtained by connecting the two
oxygen atoms).

[0053]Here, the chain polymer to be used in the present invention is a
polymer having a linear main chain, and may partially have a short
branched chain. Generally, the orientation of a chain polymer is given by
drawing a film. Therefore, it can be said that in the case of uniaxial
drawing, the orientation direction of the polymer is equal to a direction
in which the film is drawn, and that in the case of biaxial drawing, the
orientation direction of the polymer is equal to a direction in which the
film is mainly drawn.

[0054]Further, the term "two oxygen atoms are bonded to atoms constituting
a main chain, respectively" described with reference to the side chain
(a) means that one oxygen atom is bonded to one atom constituting a main
chain and the other oxygen atom is bonded to another atom constituting a
main chain. The term "aromatic group" means an aromatic compound group
having π electrons arranged in a planar cyclic array. Examples of such
an aromatic group include a benzene ring, compounds obtained by
condensing two or more benzene rings, and heteroaromatic compounds
containing an atom other than carbon. The term "the aromatic group(s) is
(are) arranged in a direction such that the planar structure thereof is
substantially orthogonal to a virtual line obtained by connecting the two
oxygen atoms" means that in a case where a virtual line is created by
connecting the two oxygen atoms, the aromatic group(s) is (are) arranged
in such a manner that a line parallel to the virtual line is
substantially orthogonal to the planar structure of the aromatic
group(s). That is, the term does not mean that the virtual line itself
obtained by connecting the two oxygen atoms intersects with the planar
structure of the aromatic group(s).

[0055]When the polymer is oriented, two oxygen atoms are arranged along a
direction in which the main chain of the polymer is oriented. As
described above, in the polymer, the aromatic group(s) of the side chain
(a) is (are) arranged in a direction substantially orthogonal to a
virtual line obtained by connecting the two oxygen atoms. That is, the
planar structure of the aromatic group(s) is arranged in a direction
substantially orthogonal to a direction in which the main chain of the
polymer is oriented (where it can be considered that the planar structure
of the aromatic group(s) of the side chain (a) is not accurately arranged
at 90° to a direction in which the main chain of the polymer is
oriented, and is in fact arranged at about 75 to 105° to a
direction in which the main chain of the polymer is oriented). It can be
considered that the existence of the side chain (a) arranged in such a
manner described above allows the retardation film of the present
invention to have wavelength dispersion characteristics reverse to those
of a retardation film formed of a conventional polymer, that is,
wavelength dispersion characteristics such that the in-plane retardation
thereof is smaller at a shorter wavelength lying within a visible light
wavelength range, and is larger at a longer wavelength lying within a
visible light wavelength range.

[0056]Specific examples of an alkyl group having 1 to 8 carbon atoms
represented by R1 or R2 in the general formula (I) include
methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl,
t-butyl, n-pentyl, isopentyl, n-hexyl, and 2-ethyl hexyl groups.

[0057]Examples of the side chain (a) represented by the general formula
(I) in which either R1 or R2 is an aromatic group include those
represented by the following general formulas (II) and (III):

[0058]wherein two oxygen atoms are bonded to atoms constituting a main
chain, respectively; R3 represents a hydrogen atom or an alkyl group
having 1 to 8 carbon atoms; R4 and R8 each independently
represent a hydrogen atom, a linear or branched alkyl group having 1 to 4
carbon atoms, a linear or branched alkoxyl group having 1 to 4 carbon
atoms, a linear or branched thioalkoxyl group having 1 to 4 carbon atoms,
halogen, a nitro group, an amino group, a hydroxyl group, or a thiol
group (where R4 and R8 are not simultaneously hydrogen atoms);
and R5, R6, and R7 each independently represent a hydrogen
atom or a substituent,

[0059]wherein two oxygen atoms are bonded to atoms constituting a main
chain, respectively; R3 represents a hydrogen atom or an alkyl group
having 1 to 8 carbon atoms; and A represents a naphthyl group which may
have a substituent, an anthranyl group which may have a substituent, or a
phenanthrenyl group which may have a substituent (where one or more
carbon atoms constituting the naphthyl, anthranyl, or phenanthrenyl group
may be substituted by one or more nitrogen atoms).

[0060]As described above, in the side chain (a) having a structure
represented by the general formula (II), R4 and R8 in the ortho
position of the benzene ring are not simultaneously hydrogen atoms, and
at least either R4 or R8 is substituted by a substituent such
as an alkyl group. Such introduction of a substituent(s) in the ortho
position causes larger steric hindrance between the substituent(s) and
the oxygen atoms. As a result, the substituent(s) is (are) arranged
between the two oxygen atoms. This can be regarded as the reason why the
side chain (a) represented by the general formula (II) is arranged in a
direction such that the planar structure of the benzene ring thereof is
substantially orthogonal to a virtual line obtained by connecting the two
oxygen atoms.

[0061]Further, as described above, the side chain (a) represented by the
general formula (III) has an aromatic group in which two or more benzene
rings are condensed. Such a condensed ring aromatic group is sterically
bulky due to a benzene ring(s) condensed to a benzene ring bonded to
--OCO--, thereby causing larger steric hindrance between the condensed
ring aromatic group and the oxygen atoms. This can be regarded as the
reason why the side chain (a) represented by the general formula (III) is
arranged in a direction such that the planar structure of the aromatic
group thereof is substantially orthogonal to a virtual line obtained by
connecting the two oxygen atoms.

[0062]Specific examples of an alkyl group having 1 to 8 carbon atoms
represented by R3 in the general formula (II) include methyl, ethyl,
n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, t-butyl, n-pentyl,
isopentyl, n-hexyl, 2-ethyl hexyl groups. Specific examples of an alkyl
group having 1 to 4 carbon atoms represented by R4 and R8 in
the general formula (II) include methyl, ethyl, n-propyl, isopropyl,
n-butyl, isobutyl, sec-butyl, and t-butyl groups. Specific examples of an
alkoxyl group having 1 to 4 carbon atoms represented by R4 and
R8 in the general formula (II) include methoxy, ethoxy, n-propoxy,
isopropoxy, n-butoxy, and isobutoxy groups. Examples of a substituent
represented by R5, R6, and R7 in the general formula (II)
include those mentioned below as substituents in A in the general formula
(III).

[0063]In the side chain (a) represented by the general formula (II),
R3 is preferably a hydrogen atom (which is sterically small) because
the planar structure of the aromatic group is easily arranged in a
direction substantially orthogonal to a virtual line obtained by
connecting the two oxygen atoms, due to steric hindrance between the
substituent(s) in the ortho position and the oxygen atoms. Particularly,
it is preferred that in the side chain (a) represented by the general
formula (II), R3 is a hydrogen atom, and R4 and R8 are
each independently a linear or branched alkyl group having 1 to 4 carbon
atoms, a linear or branched alkoxyl group having 1 to 4 carbon atoms, a
linear or branched thioalkoxyl group having 1 to 4 carbon atoms, halogen,
a nitro group, an amino group, a hydroxyl group, or a thiol group (that
is, neither R4 nor R8 is a hydrogen atom), because the planar
structure of the aromatic group is more easily arranged in a direction
substantially orthogonal to a virtual line obtained by connecting the two
oxygen atoms. It is more preferred that in the side chain (a) represented
by the general formula (II), R3 is a hydrogen atom, and R4 and
R8 are each independently a linear or branched alkyl group having 1
to 4 carbon atoms, a linear or branched alkoxyl group having 1 to 4
carbon atoms, or halogen. In addition to the reason described above, from
the viewpoint of ease of introduction of an acetal structure and
stability of the acetal structure, it is particularly preferred that in
the side chain (a) represented by the general formula (II), R3,
R5, and R7 are each a hydrogen atom, and R4, R6, and
R8 are each a methyl group.

[0064]In a case where a naphthyl, anthranyl, or phenanthrenyl group
represented by A in the general formula (III) has a substituent, the
substituent is not particularly limited. Examples of such a substituent
include a linear or branched alkyl or alkoxyl group having 1 to 8 carbon
atoms, a cycloalkyl or cycloalkoxyl group having 3 to 6 carbon atoms, a
hydroxyl group, a carboxyl group, an amino group, halogen, a nitro group,
a thiol group, an aldehyde group, a cyano group, or a sulfonic acid
group. The number of such substituents in the naphthyl, anthranyl, or
phenanthrenyl group represented by A may be one or two or more. In a case
where two or more substituents are present in the naphthyl, anthranyl, or
phenanthrenyl group represented by A, these substituents are the same or
different.

[0065]In the side chain (a) represented by the general formula (III), A is
preferably a 9-anthranyl group which may have a substituent because the
planar structure of the benzene rings of the 9-anthranyl group is easily
arranged in a direction substantially orthogonal to a virtual line
obtained by connecting the two oxygen atoms, due to steric hindrance
between the benzene rings and the oxygen atoms. It is to be noted that
the term "which may have a substituent" means that the group is
non-substituted or substituted by one or more substituents.

[0066]An example of the side chain (a) represented by the general formula
(I) in which R1 and R2 are both aromatic groups includes one
represented by the following general formula (XIII):

[0067]wherein R10 to R19 each independently represent a hydrogen
atom or a linear or branched alkyl group having 1 to 4 carbon atoms
(where R10 and R14 are not simultaneously hydrogen atoms and
R15 and R19 are not simultaneously hydrogen atoms); and two
benzene rings may be partially bonded together through a single bond.

[0068]Specific examples of the side chain (a) represented by the general
formula (XIII) include those having a structure represented by the
following formula (XIV):

[0069]The polymer of the present invention should have at least any one of
the groups mentioned above as the side chain (a) to be bonded to the main
chain thereof. For example, the polymer of the present invention may
have, as the side chain (a), both the groups represented by the general
formulas (II) and (III) bonded to the main chain thereof.

[0070]From the viewpoint of enabling the retardation film of the present
invention to reliably exhibit reverse wavelength dispersion
characteristics, the amount of the side chain (a) to be introduced into
the main chain of the polymer is preferably 1 mol % or more, more
preferably 5 mol % or more, with respect to the total amount of side
chains of the polymer. Further, from the viewpoint of enabling the
retardation film of the present invention to have a positive
birefringence anisotropy, the amount of the side chain (a) to be
introduced into the main chain of the polymer is preferably 50 mol % or
less, more preferably 30 mol % or less, with respect to the total amount
of side chains of the polymer.

[0071]As described above, the chain polymer to be used for the retardation
film of the present invention has the side chain (a), but may further
have a side chain other than the side chain (a). Examples of such a side
chain include, but are not limited to, a hydroxyl group, an amino group,
a thiol group, an alkoxyl group, halogen, a cyano group, a nitro group,
an ester group, a ketone group, an aldehyde group, an amide group, a
urethane group, a urea group, a carbonate group, and a group represented
by the following general formula (IV):

[0072]wherein R9 represents a hydrogen atom or a linear, branched or
cyclic alkyl group (where one or more carbon atoms of the alkyl group may
be substituted by one or two or more non-adjacent oxygen atoms).

[0073]Among these side chains other than the side chain (a), from the
viewpoint of improving reverse wavelength dispersion characteristics and
solubility of the chain polymer in a solvent used for film formation, the
chain polymer to be used for the retardation film of the present
invention preferably has a hydroxyl group as a side chain (b) other than
the side chain (a).

[0074]Further, from the viewpoint of improving transparency and decreasing
glass transition temperature, the chain polymer to be used for the
retardation film of the present invention preferably has a group
represented by the general formula (IV) as a side chain (c) other than
the side chain (a). Among the groups represented by the general formula
(IV), at least one selected from groups represented by the general
formula (IV) in which R9 is a hydrogen atom or a linear, branched or
cyclic alkyl group having 1 to 12 carbon atoms (where one or more carbon
atoms of the alkyl group may be substituted by one or two or more
non-adjacent oxygen atoms) is preferably introduced as a side chain (c)
into the main chain of the chain polymer. Among such groups, a group
represented by the following formula (XV) is more preferred:

[0075]The amount of the side chain (b) and/or the side chain (c) to be
introduced into the main chain of the chain polymer can be appropriately
adjusted within the range of the remaining amount after deduction of the
amount of the side chain (a) in view of desired characteristics of the
retardation film. However, from the viewpoint of improving reverse
wavelength dispersion characteristics and the solubility of the chain
polymer in a solvent, the amount of a hydroxyl group as the side chain
(b) is preferably 5 mol % or more, more preferably 20 mol % or more, with
respect to the total amount of side chains of the chain polymer. The
upper limit amount of the side chain (b) is appropriately adjusted
depending on the amount of the side chain (a) and the side chain (c), but
is preferably about 95 mol % or less, more preferably about 80 mol % or
less. Further, from the viewpoint of improving transparency, the amount
of the side chain (c) to be introduced into the main chain of the chain
polymer is preferably 1 mol % or more, more preferably 5 mol % or more,
with respect to the total amount of side chains of the chain polymer. The
upper limit amount of the side chain (c) is preferably about 90 mol % or
less, more preferably about 50 mol % or less.

[0076]As described above, the chain polymer to be used for the retardation
film of the present invention has the side chain (a) and, if necessary,
the side chain (b) and/or the side chain (c) other than the side chain
(a). The main chain of the chain polymer is not particularly limited, and
can have any structure as long as it exhibits birefringence and does not
impair the object of the present invention. It is to be noted that in the
present invention, the term "main chain" means a long-chain portion in
which repeating units each having some kind of structure formed by atoms
constituting the main chain are bonded together. The main chain may have
a branched chain.

[0077]An example of such a main chain includes a polymer having a
repeating unit represented by the following general formula (XVI):

[0079]Among these main chains, from the viewpoint of ease of introduction
of the side chain (a) and, if necessary, the side chain (b) and/or the
side chain (c) and stability, a polymer having a repeating unit
represented by the general formula (XVI) in which E1 and E2 are
each a single bond is preferred. Also, a polymer having a repeating unit
represented by the general formula (XVI) in which at least G is an
alkylene group is preferred.

[0080]Among polymers in which the main chain thereof has an alkylene group
and the side chain (a) is bonded to the main chain, a polymer having as a
repeating unit (A), at least either a structure represented by the
following general formula (V) or a structure represented by the following
general formula (VI):

[0081]wherein R3 represents a hydrogen atom or an alkyl group having
1 to 8 carbon atoms; R4 and R8 each independently represent a
hydrogen atom, a linear or branched alkyl group having 1 to 4 carbon
atoms, a linear or branched alkoxyl group having 1 to 4 carbon atoms, a
linear or branched thioalkoxyl group having 1 to 4 carbon atoms, halogen,
a nitro group, an amino group, a hydroxyl group, or a thiol group (where
R4 and R8 are not simultaneously hydrogen atoms); and R5,
R6, and R7 each independently represent a hydrogen atom or a
substituent,

[0082]wherein R3 represents a hydrogen atom or an alkyl group having
1 to 8 carbon atoms; and A represents a naphthyl group which may have a
substituent, an anthranyl group which may have a substituent, or a
phenanthrenyl group which may have a substituent (where one or more
carbon atoms constituting the naphthyl, anthranyl, or phenanthrenyl group
may be substituted by one or more nitrogen atoms).

[0083]In such a polymer having the repeating unit (A) represented by the
general formula (V), two oxygen atoms are arranged along a direction in
which the constituent atoms (carbon atoms) of the main chain to which
these oxygen atoms are bonded are oriented. Further, in a case where
R4 or/and R8 in the ortho position of the benzene ring is (are)
a substituent(s), steric hindrance between the substituent(s) and the
oxygen atoms becomes larger so that the substituent(s) in the ortho
position is (are) arranged between the two oxygen atoms. This can be
regarded as the reason why the planar structure of the benzene ring is
arranged in a direction substantially orthogonal to a virtual line
obtained by connecting the two oxygen atoms. For the same reason, also in
the case of a polymer having the repeating unit (A) represented by the
general formula (VI), it can be considered that the planar structure of
the condensed ring aromatic group is arranged in a direction
substantially orthogonal to a virtual line obtained by connecting the two
oxygen atoms.

[0084]The repeating units represented by the general formulas (V) and (VI)
have the side chains represented by the general formulas (II) and (III),
respectively. Therefore, preferred selections of R3 to R8 and A
and specific examples of substituents in the general formulas (V) and
(VI) are the same as mentioned above with reference to the general
formulas (II) and (III). For example, R3 in the general formula (V)
is preferably a hydrogen atom. It is also preferred that R4 and
R8 in the general formula (V) are each independently a substituent
selected from the substituents mentioned above (that is, neither R4
nor R8 is a hydrogen atom). It is to be noted that a repeating unit
represented by the general formula (V) in which neither R4 nor
R8 is a hydrogen atom is one represented by the general formula (V')
of claim 15. In this case, it is preferred that R4 and R8 are
each independently a linear or branched alkyl group having 1 to 4 carbon
atoms or halogen such as a chlorine atom. More specifically, a polymer
having the repeating unit (A) represented by the general formula (V) in
which R3, R5, and R7 are each a hydrogen atom and R4
and R8 are each a methyl group is preferred (it is to be noted that
R6 may be either a hydrogen atom or a methyl group, but is
preferably a methyl group).

[0085]Further, from the viewpoint of improving the reverse wavelength
dispersion properties of the retardation film and the solubility of the
chain polymer in a solvent used for film forming, a polymer having, in
addition to the repeating unit (A), a repeating unit (B) represented by
the following general formula (VII) is also preferred:

[0086]Furthermore, from the viewpoint of improving transparency and
decreasing a glass transition temperature, a polymer having, in addition
to the repeating units (A) and (B), a repeating unit (C) represented by
the following general formula (VIII) is also preferred:

[0087]wherein R9 represents a hydrogen atom or a linear, branched, or
cyclic alkyl group having 1 to 12 carbon atoms (where one or more carbon
atoms of the alkyl group may be substituted by one or two or more
non-adjacent oxygen atoms).

[0088]The repeating units (A) and (B) and, if necessary, (C) may be
arranged in either block or random fashion. It is to be noted that the
chain polymer may have other repeating units in addition to the repeating
unit (A) and, if necessary, the repeating unit (B) and the repeating unit
(C), as long as the object of the present invention is not impaired.

[0089]The chain polymer to be used for forming the retardation film of the
present invention is selected from polymers in which the side chain (a)
and, if necessary, the side chain (b) and/or the side chain (c) are
bonded to the main chain thereof. Further, the chain polymer has the
repeating unit (A) and, if necessary, the repeating unit (B) and the
repeating unit (C). As described above, examples of the chain polymer to
be used for forming the retardation film of the present invention include
the various polymers as described above. Among them, a polymer having at
least any one of the structures represented by the following general
formulas (IX), (X), (XI), and (XII) is most preferred:

[0090]wherein l is 5 to 30 mol %; m is 20 to 80 mol %; and n is 1 to 70
mol %,

[0091]wherein l is 5 to 30 mol %; m is 20 to 80 mol %; and n is 1 to 70
mol %,

[0092]wherein l is 5 to 30 mol %; m is 20 to 80 mol %; and n is 1 to 70
mol %, and

[0093]wherein l is 5 to 30 mol %; m is 20 to 60 mol %; n is 20 to 60 mol
%; and o is 1 to 55 mol %.

[0094]As described above, the retardation film of the present invention is
formed from a single-layer film, and therefore has a thickness smaller
than that of a conventional retardation film having a laminate structure.
Further, the retardation film of the present invention has an in-plane
retardation of about λ/2 or λ/4 for light having a wavelength
of λ lying within almost the entire visible light wavelength range
from 400 to 700 nm, and therefore provides substantially the same form of
polarization at any wavelength from 400 to 700 nm. For example, even when
white light enters the retardation film of the present invention, the
incident white light is not converted to colored polarized light, thereby
enabling white polarized light to be obtained. Furthermore, the
retardation film of the present invention is excellent in transparency.
The chain polymer to be used for forming the retardation film of the
present invention exhibits good solubility in a solvent used for film
formation, and has a glass transition temperature within an appropriate
range, which produces various effects such as improving heat resistance
of the retardation film of the present invention and enabling Z drawing.

[0095]Next, a method for producing the above-described chain polymer will
be described.

[0096]A method for producing the polymer to be used for forming the
retardation film of the present invention is not particularly limited,
and various methods can be employed.

[0097]Among various methods, a method in which a raw polymer whose main
chain has hydroxyl groups is reacted with specific aromatic aldehyde or
aromatic ketone is preferably employed because a polymer in which the
side chain (a) is introduced into the main chain thereof can be
relatively easily obtained. Particularly, from the viewpoint of stability
in introduction of the side chain (a), relatively small wavelength
dispersion, and general versatility, the polymer is preferably produced
using polyvinyl alcohol. More specifically, the polymer is preferably
produced by reacting hydroxyl groups in polyvinyl alcohol used as a raw
polymer with specific aromatic aldehyde or aromatic ketone to carry out
acetalization (that is, introduction of a structure represented by
RCH(OR)(OR)) or ketalization (that is, introduction of a structure
represented by RRC(OR)(OR)).

[0098]In this regard, it is to be noted that when polyvinyl alcohol is
reacted with a specific aromatic aldehyde, an acetal structure is
introduced into the main chain thereof as a side chain so that a polymer
having a structure represented by the general formula (V) or (VI) in
which R3 is a hydrogen atom is obtained. Likewise, when polyvinyl
alcohol is reacted with specific aromatic ketone, a ketal structure is
introduced into the main chain thereof as a side chain so that a polymer
having a structure represented by the general formula (V) or (VI) in
which R3 is an alkyl group is obtained.

[0099]For example, in order to obtain a polymer having the repeating unit
(A) represented by the general formula (V), polyvinyl alcohol is reacted
with benzaldehyde or acetophenone having a substituent in at least one
ortho position under acid conditions. Specific examples of such
benzaldehyde or acetophenone having a substituent in at least one ortho
position include 2,4,6-trimethylbenzaldehyde (mesitaldehyde),
2,4,6-triethylbenzaldehyde, 2,6-dimethylbenzaldehyde,
2-methylbenzaldehyde, 2-methylacetophenone, and 2,4-dimethylacetophenone.

[0100]Likewise, in order to obtain a polymer having the repeating unit (A)
represented by the general formula (VI), polyvinyl alcohol is reacted
with condensed ring aromatic aldehyde or condensed ring aromatic ketone.
Specific examples of such condensed ring aromatic aldehyde or condensed
ring aromatic ketone include substituted 1-naphthoaldehyde, substituted
2-naphthoaldehyde, 9-anthraldehyde, substituted 9-anthraldehyde, and
acetonaphthone.

[0101]In such a production method, by controlling the amount of aromatic
aldehyde or aromatic ketone to be reacted with polyvinyl alcohol, it is
possible to obtain a polymer having the repeating unit (A) and the
repeating unit (B) because some hydroxyl groups in polyvinyl alcohol are
substituted by aromatic groups but other hydroxyl groups remain
unsubstituted.

[0102]Further, by simultaneously and/or sequentially acetalizing polyvinyl
alcohol with aromatic aldehyde or aromatic ketone and a saturated
aliphatic aldehyde having 1 to 12 carbon atoms (e.g., propionaldehyde,
acetaldehyde), formaldehyde, or alicyclic aldehyde, it is possible to
obtain a polymer having the repeating unit (A) and the repeating unit
(C). In this case, by further controlling the amount of aromatic aldehyde
or aromatic ketone to be reacted with polyvinyl alcohol and the amount of
saturated aliphatic aldehyde, formaldehyde, or alicyclic aldehyde to be
reacted with polyvinyl alcohol, it is possible to obtain a polymer having
the repeating units (A) to (C) because some hydroxyl groups remain
unsubstituted.

[0103]The degree of polymerization of the polymer is not particularly
limited as long as it is at a level suitably used for, for example, a
retardation film. However, from the viewpoint of achieving film strength
high enough to withstand drawing, the degree of polymerization of the
polymer is preferably in the range of about 100 to 20,000, more
preferably in the range of about 500 to 10,000. The degree of
polymerization of the polymer can be controlled by appropriately changing
the kind of main chain of the polymer or changing the kind or amount of
the side chain (a) and the like.

[0104]The glass transition temperature of the polymer varies depending on
the kind of main chain of the polymer or the kind or amount of each of
the side chains (a) to (c), but is in the range of, for example, about 80
to 180° C. Therefore, the polymer having such a glass transition
temperature has heat resistance high enough to use as a retardation film.
Further, unlike a conventional polymer having a high glass transition
temperature exceeding about 200° C., the polymer of the present
invention has an appropriate glass transition temperature, and therefore
can be subjected to not only uniaxial drawing but also Z drawing
according to a conventionally known method.

[0105]Next, a method for forming a film of the polymer to obtain a
retardation film will be described.

[0106]In this regard, it is to be noted that in this specification, the
term "film" includes one generally called "sheet".

[0107]The retardation film of the present invention consists of a
single-layer film formed of the polymer described above. A method for
forming such a film is not particularly limited, and therefore the
polymer can be formed into a film by, for example, a casting method, a
melt extrusion method, or a calender method. Among these methods, from
the viewpoint of obtaining an optically uniform film with higher
thickness accuracy, a casting method is preferably employed.

[0108]A casting method usually uses a solvent for dissolving the polymer.
In connection with this, there is a case where the polymer of the present
invention having a hydroxyl group as a side chain (b) exhibits good
solubility in a solvent which cannot be used when a conventional
retardation film comprising a polymer film is formed by a casting method.
Examples of such a solvent for dissolving the polymer of the present
invention include tetrahydrofuran, dimethylsulfoxide,
N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone,
cyclopentanone, cyclohexanone, methyl ethyl ketone, ethyl acetate,
dichloromethane, and toluene. It is to be noted that, if necessary, the
solvent may be heated for dissolving the polymer therein.

[0109]By drawing the thus obtained film, it is possible to obtain a
retardation film of the present invention. Examples of a drawing method
include, but are not limited to, conventional uniaxial drawing methods
such as tenter drawing, inter-roll drawing, and inter-roll compression
drawing; and conventional biaxial drawing methods such as simultaneous
biaxial drawing using a whole tenter technique and successive biaxial
drawing using a roll tenter technique.

[0110]Among biaxial drawing methods, Z drawing can also be carried out in
the present invention depending on the kind (characteristics) of a
polymer to be used for forming a film. Z drawing can be carried out by,
for example, a conventional Z drawing method such that drawing stress is
generated in a thickness direction (that is, in the Z-axis direction) by
shrinking a film in a direction (that is, in the Y-axis direction) that
is orthogonal to or is intersecting with the direction of heat drawing of
the film (that is, to or with the X-axis direction).

[0111]It is to be noted that for the purpose of improving the drawing
property of the film, the film may contain one or more plasticizers such
as phthalic esters (e.g., dimethyl phthalate, diethyl phthalate, dibutyl
phthalate); phosphoric esters (e.g., trimethyl phosphate, triethyl
phosphate, triphenyl phosphate); and fatty acid esters (e.g., diethyl
adipate, dibutyl fumarate). The amount of the plasticizer to be added to
the film is preferably about 1 to 20 parts by weight with respect to 100
parts by weight of the polymer in view of the effect of improving drawing
property and the influence of the plasticizer on wavelength dispersion of
a resultant retardation film. In addition to such plasticizers, other
additives such as antioxidants and ultraviolet absorbers may be
appropriately added to the film in accordance with the intended use of
the film.

[0112]The conditions for drawing the film, such as temperature and drawing
ratio vary depending on the kind of main chain constituting the polymer,
the kind or amount of the side chain (a), and the kind or amount of the
side chain (b) and/or the side chain (c) introduced into the main chain
if necessary, and are therefore appropriately determined. However, for
example, the drawing temperature is preferably about 50 to 200°
C., and the drawing ratio is preferably about 1.1 to 4.0 times. The thus
obtained retardation film of the present invention has excellent
transparency because the visible light transmittance and haze value
thereof measured in accordance with JIS K 7105 are about 88 to 93% and
about 0.1 to 3%, respectively. Further, the thickness of the retardation
film is sufficiently small because it is usually in the range of about 20
to 200 μm, preferably in the range of about 40 to 100 μm.

[0113]As described above, the retardation film of the present invention
consists of a single-layer film of the chain polymer having the side
chain (a), and exhibits reverse wavelength dispersion characteristics.
Such a retardation film can exhibit, by itself, wavelength dispersion
characteristics such that retardation for light having a wavelength of
λ lying within almost the entire visible light wavelength range is,
for example, about λ/2 or λ/4, thereby enabling conversion to
white polarized light. As described above, the retardation film of the
present invention has reverse wavelength dispersion characteristics.
Although the wavelength dispersion of the retardation film of the present
invention varies depending on the kind of main chain constituting a
polymeric material of the film, the kind or amount of the side chain (a),
and the kind or amount of the side chain (b) introduced into the main
chain if necessary, when retardations are measured under the following
conditions, they roughly satisfy the relation:
Re(450)/Re(550)≦0.97, Re(650)/Re(550)≧1.01. Here, Re(450),
Re(550), and Re(650) represent in-plane retardations measured at
wavelengths 450, 550 and 650 nm, respectively.

[0114]In a case where the retardation film of the present invention
comprises a film obtained by, for example, uniaxial drawing, the
refractive indexes of the film satisfy the relation represented by the
formula: nx>ny=nz, where nx, ny, and nz represent a refractive index
in the direction of drawing (that is, in the direction of the X-axis (in
a direction such that an in-plane refractive index is greatest), a
refractive index in a direction orthogonal to the drawing direction (that
is, in the direction of the Y-axis), and a refractive index in the
direction of a thickness (that is, in the direction of the Z-axis),
respectively. On the other hand, in a case where the retardation film of
the present invention is obtained by the Z drawing described above among
various biaxial drawing methods, the refractive indexes of the
retardation film satisfy the relation represented by the formula:
nx>nz>ny. Meanwhile, as a parameter of visual characteristics, a
value Nz represented by the formula: Nz=(nx-nz)/(nx-ny) is often used.
When the value Nz is 0.5, retardation is not changed according to an
angle of inclination even in a case where the retardation film is
inclined in the direction of a slow axis (that is, in the direction of
the X-axis). That is, it has become apparent that by setting the value Nz
to 0.5, it is possible to remove the viewing angle dependence of
retardation. From the viewpoint of improving visual characteristics, it
is desired that drawing of the film is carried out while paying
particular attention to the control of three dimensional refractive
indexes.

[0115]When light having a certain wavelength enters the retardation film
of the present invention, a refractive index difference ne-no
(ne represents an extraordinary index and no represents an
ordinary index) is 0.005 or less at 550 nm (hereinafter, ne-no
is simply referred to as "Δn"). However, in a case where a film
formed using a mixture of the chain polymer of the present invention and
a liquid crystalline compound is used as a retardation film, the value of
Δn of the retardation film can be made 0.01 or more because a film
itself formed of a liquid crystalline compound generally has a relatively
high value of Δn.

[0116]When the value of Δn of the retardation film is high, it
becomes possible to decrease the thickness of the retardation film while
achieving a desired retardation. Particularly, it is possible to obtain a
retardation film which has a thickness of 100 μm or less and gives a
retardation of λ/2 to light having any wavelength lying within
almost the entire visible light wavelength range.

[0117]The liquid crystalline compound is not particularly limited as long
as the object of the present invention is not impaired. Examples of such
a liquid crystalline compound include liquid crystalline low molecular
weight compounds such as azomethines, azoxys, cyanobiphenyls, cyanophenyl
esters, benzoic acid esters, cyclohexanecarboxylic acid phenyl esters,
cyanophenylcyclohexanes, cyano-substituted phenylpyrimidines,
alkoxy-substituted phenylpyrimidines, phenyldioxanes, tolans, and
alkenylcyclohexylbenzonitriles; and liquid crystalline monomers obtained
by introducing a polymerizable group such as a (meth)acryloyloxy group,
an epoxy group, a vinyloxy group, a propargyl group, or an isocyanate
group into the molecular end of the above-mentioned liquid crystalline
low molecular weight compounds. Among these liquid crystalline compounds,
one or more of them are appropriately selected so that a retardation film
having a desired value of Δn can be obtained or according to the
kind of polymer to be used. From the viewpoint of sufficiently exhibiting
the effect of increasing the value of Δn, the amount of the liquid
crystalline compound to be used is preferably 1 part by weight or more,
more preferably 3 parts by weight or more, with respect to 100 parts by
weight of the chain polymer. Further, from the viewpoint of maintaining
the characteristics such that retardation is larger at a longer
wavelength, the amount of the liquid crystalline compound to be used is
preferably 20 parts by weight or less, more preferably 10 parts by weight
or less, with respect to 100 parts by weight of the chain polymer.

[0118]The retardation film of the present invention can be used as an
optical film by laminating it with other optical materials. For example,
a laminate film obtained by laminating the retardation film of the
present invention to a polarizing plate can be used as an optical film
such as an elliptically polarizing plate or a circularly polarizing
plate. Further, it is possible to laminate the retardation film of the
present invention adjusted to have a retardation of λ/4 and the
retardation film of the present invention adjusted to have a retardation
of λ/2 to a polarizing plate to obtain an optical film. The
retardation film of the present invention can be directly laminated to a
polarizer of a polarizing plate or can be laminated to a polarizer of a
polarizing plate through a protective film. On the surface of the
retardation film of the present invention, it is possible to provide a
pressure-sensitive adhesive layer with which the retardation film is
adhered to other components such as a liquid crystal cell. In a case
where the surface of the pressure-sensitive adhesive layer is exposed,
the pressure-sensitive adhesive layer is preferably covered with a
release sheet. As described above, by laminating the retardation film of
the present invention with other optical materials having different
functions, it is possible to obtain various optical films such as an
optical film in which a polarizing plate, the retardation film of the
present invention, and a band-pass filter are laminated in this order and
an optical film having a protective film on its surface.

[0119]The retardation film of the present invention or the optical film
having a laminate structure comprising the retardation film of the
present invention can be suitably used as a structural component of
various image display devices such as liquid crystal displays. For
example, in the case of a liquid crystal display, the optical film is
provided on one surface or both surfaces of a liquid crystal cell, and
the type of such a liquid crystal display can be appropriately selected
from, for example, conventional transmissive, reflective, and
transflective types. Therefore, the liquid crystal display can be formed
from any liquid crystal cell. For example, a liquid crystal cell of a
passive matrix driving type typified by a liquid crystal cell of a thin
film transistor type can also be used. In a case where the optical film
of the present invention is provided on both surfaces of a liquid crystal
cell, the optical films are the same or different. Further, when the
liquid crystal display is manufactured, one or more sheet materials such
as a prism array sheet, a lens array sheet, a diffuser, and a backlight
can be arranged at their appropriate positions.

EXAMPLES

[0120]Hereinbelow, the retardation film of the present invention will be
described in more detail with reference to the following Examples.
However, the present invention is not limited thereto.

[0121]Conditions for measuring various properties of polymers and
retardation films are as follows.

[0122](Measurement of Composition Ratio)

[0123]200 mg of a polymer as a measuring object was sampled, and the
polymer, 0.93 g of imidazole and 1.37 g of t-butyldimethylchlorosilane
were added to 10 mL of THF at room temperature under nitrogen, and they
were stirred at room temperature for 12 hours. After the completion of
the reaction, the reaction mixture was purified by reprecipitation three
times with methanol. The thus obtained polymer was analyzed by
1H-NMR (CDCl3 solvent).

[0124]In Production Examples 1 and 2, the composition ratio of an obtained
polymer was determined from peaks appearing at around 0 ppm, 0.8 ppm, and
6.8 ppm.

[0125]In Production Example 3, the composition ratio of an obtained
polymer was determined from peaks appearing at around 0 ppm, 3.3-5.4 ppm,
and 6.8 ppm.

[0126]In Production Example 4, the composition ratio of an obtained
polymer was determined from peaks appearing at around 0 ppm, 3.5-5.0 ppm,
and 6.8 ppm.

[0127](Measurement of Glass Transition Temperature)

[0128]The glass transition temperature of a polymer was measured using a
differential scanning thermometer (manufactured by SEIKO under the trade
name of "DSC6200"). Measurement was carried out by increasing the
temperature of a powder sample of the polymer at a rate of 10°
C./min from room temperature while flowing nitrogen gas at 80 mL/min. The
measurement was carried out twice, and measured data obtained for the
second time was used. In each measurement, the amount of the powder
sample used was 3 mg. The temperature calibration of the thermometer was
carried out using reference materials (indium and tin).

[0129](Rate of Retardation Change)

[0130]A sample film was placed in a dryer set at 80° C. for 10
hours, and then the retardation of the sample film was measured. The rate
of retardation change was determined from the retardation of the sample
film measured prior to this test and the retardation of the sample film
measured after the treatment at 80° C. for 10 hours.

[0131](Measurement of IN-Plane Retardation)

[0132]The in-plane retardation of a sample film was measured using
"KOBRA21-ADH" manufactured by Oji Scientific Instruments.

[0133](Measurement of Thickness)

[0134]The thickness of a sample film was measured using a micrometer
(manufactured by MITUTOYO).

[0135](Measurement of Water Absorption)

[0136]The water absorption of a sample film was measured in accordance
with JIS K 7209 "Testing Methods for Water and Boiling Water Absorption
of Plastics". The sample film had a size of 50 mm×50 mm and a
thickness of 40 to 100 μm.

[0137](Analysis by Polarized IR)

[0138]A sample film was analyzed by polarized IR at room temperature with
the use of FT/IR-230 (manufactured by JASCO Corporation) equipped with
PL-82 as an IR polarizer.

Production Example 1

[0139]5.0 g of PVA with a degree of polymerization of 1800 (manufactured
by Nippon Synthetic Chemical Industry under the trade name of "NH-18")
dried at 105° C. for 2 hours was dissolved in 95 mL of DMSO. Then,
3.78 g of mesitaldehyde, 1.81 g of propionaldehyde, and 1.77 g of
p-toluenesulfonic acid monohydrate were added thereto, and they were
stirred at 40° C. for 4 hours. Reprecipitation was carried out
using a water/methanol mixture (water/methanol=2/1) in which 2.35 g of
sodium bicarbonate was dissolved. The reprecipitated polymer was
separated by filtration, and was then dissolved in THF. Then,
reprecipitation was carried out using diethyl ether. The reprecipitated
polymer was separated by filtration and dried. As a result, 7.89 g of a
white polymer was obtained. The thus obtained polymer was analyzed under
the measurement conditions described above. As a result, it was found
that the polymer was a novel polymer having a structure represented by
the formula (IX) in which the ratio between the units of vinylmesital,
vinylpropional, and vinylalcohol was 22:46:32. The glass transition
temperature of the polymer was 102° C.

Example 1

[0140]The polymer obtained in the Production Example 1 was dissolved in
DMF, and was then formed into a film with the use of an applicator and
dried. The dried film was drawn to 1.8 times at 110° C. using a
drawing machine, and as a result, a uniaxially-drawn film having a
thickness of 85 μm was obtained. The thus obtained drawn film had
reverse wavelength dispersion characteristics such that Re(450), Re(550),
and Re(650) were 116.4 nm, 138.1 nm, and 147.6 nm, respectively. The rate
of retardation change of the film was measured to evaluate heat
resistance, and was found to be 1% or less. The water absorption of the
film was 5%.

Production Example 2

[0141]A polymer was produced in the same manner as in the Production
Example 1 except that the amount of mesitaldehyde was changed to 2.10 g
and that the amount of propionaldehyde was changed to 2.47 g. The thus
obtained polymer was purified, and as a result, 6.48 g of a white polymer
whose ratio between the units of vinylmesital, vinylpropional, and
vinylalcohol was 18:52:30 was obtained. The glass transition temperature
of the polymer was 97° C.

Example 2

[0142]The polymer obtained in the Production Example 2 was dissolved in
DMF, and was then formed into a film with the use of an applicator and
dried. The dried film was drawn to 2 times at 110° C. using a
drawing machine, and as a result, a uniaxially-drawn film having a
thickness of 78 μm was obtained. The thus obtained drawn film had
reverse wavelength dispersion characteristics such that Re(450), Re(550),
and Re(650) were 263.9 nm, 276.8 nm, and 280.6 nm, respectively. The rate
of retardation change of the film was measured to evaluate heat
resistance, and was found to be 1% or less. The water absorption of the
film was 5%.

[0143]Further, the drawn film was analyzed by polarized IR. FIG. 1 shows a
result of plotting the relation between the angle which the polarization
direction forms with a fast axis and the intensity of an absorption peak
at 1465 cm-1 due to aromatic C═C stretching vibration. As shown
in FIG. 1, when the angle which the polarization direction formed with a
fast axis was 0° or 180°, the absorption peak intensity was
increased. On the other hand, when the angle which the polarization
direction formed with a fast axis was 90° or 270°, the
absorption peak intensity was decreased. This result indicates that the
aromatic ring of the polymer is oriented so as to be orthogonal to a
plane containing a main chain of the polymer.

Production Example 3

[0144]A polymer was produced in the same manner as in the Production
Example 1 except that the amount of mesitaldehyde was changed to 3.03 g
and that propionaldehyde was replaced with 4.30 g of 1,1-diethoxyethane.
The thus obtained polymer was purified, and as a result, 7.24 g of a
white polymer was obtained. The thus obtained polymer was analyzed under
the measurement conditions described above. As a result, it was found
that the polymer was a novel polymer having a structure represented by
the formula (X) in which the ratio between the units of vinylmesital,
vinylacetal, and vinylalcohol was 18:47:35. The glass transition
temperature of the polymer was 120° C.

Example 3

[0145]The polymer obtained in the Production Example 3 was dissolved in
DMF, and was then formed into a film with the use of an applicator and
dried. The dried film was drawn to 2 times at 155° C. using a
drawing machine, and as a result, a uniaxially-drawn film having a
thickness of 110 μm was obtained. The thus obtained drawn film had
reverse wavelength dispersion characteristics such that Re(450), Re(550),
and Re(650) were 246.7 nm, 274.1 nm, and 283.7 nm, respectively. The rate
of retardation change of the film was measured to evaluate heat
resistance, and was found to be 1% or less. The water absorption of the
film was 8%.

Production Example 4

[0146]8.8 g of the same PVA as used in the Production Example 1 was
dissolved in 500 mL of DMSO. Then, 3.0 g of mesitaldehyde, 9.0 g of
cyclohexanecarboxyaldehyde, and 3.1 g of p-toluenesulfonic acid
monohydrate were added thereto, and they were stirred at 40° C.
for 4 hours. Then, a 1N aqueous sodium hydroxide solution was added to
the reaction mixture to terminate the reaction. Reprecipitation was
carried out using water. The reprecipitated polymer was separated by
filtration, and was then dissolved in DMF. Then, reprecipitation was
carried out using diethyl ether. The reprecipitated polymer was separated
by filtration and dried. As a result, a white polymer was obtained. The
thus obtained polymer was analyzed by 1H-NMR under the conditions
described above, and as a result, it was found that mesitaldehyde and
cyclohexanecarboxyaldehyde were introduced into the PVA in the form of
acetals, and that the polymer was a novel polymer having a structure
represented by the general formula (XI) in which the ratio between the
units of vinylmesital, vinylcyclohexanecarboxal, and vinylalcohol was
10:39:51. The glass transition temperature of the polymer was 107°
C.

Example 4

[0147]The polymer obtained in the Production Example 4 was dissolved in
DMF, and was then formed into a film with the use of an applicator and
dried. The dried film was drawn to 1.5 times at 150° C. using a
drawing machine, and as a result, a uniaxially-drawn film was obtained.
The thus obtained drawn film had reverse wavelength dispersion
characteristics such that Re(450)/Re(550) and Re(650)/Re(550) were 0.934
and 1.032, respectively.

Comparative Example 1

[0148]A polymer was produced in the same manner as in the Production
Example 1 except that mesitaldehyde was replaced with 2.71 g of
benzaldehyde. The thus obtained polymer was purified, and as a result
6.02 g of a white polymer was obtained. In the same manner as in the
Example 1, the polymer was formed into a film, and the film was drawn.
The wavelength dispersion characteristics of the drawn film were
determined, and as a result, it was found that the film had wavelength
dispersion characteristics such that Re(450), Re(550), and Re(650) were
140.5 nm, 140.1 nm, and 139.5 nm, respectively and Re(450)/Re(550) and
Re(650)/Re(550) were 1.003 and 0.996, respectively. That is, the film did
not have reverse wavelength dispersion characteristics

Patent applications in class Solid polymer derived from aldehyde or aldehyde-type reactant and wherein none of the reactants forming the solid polymer contains a phenol-, amine-, -N=C=X, -N-S(=O)- or ketone group or a condensate thereof except when an amine group appears in hexamethylenetetramine or a derivative thereof (X is chalcogen)

Patent applications in all subclasses Solid polymer derived from aldehyde or aldehyde-type reactant and wherein none of the reactants forming the solid polymer contains a phenol-, amine-, -N=C=X, -N-S(=O)- or ketone group or a condensate thereof except when an amine group appears in hexamethylenetetramine or a derivative thereof (X is chalcogen)